Thermal cracking of hydrocarbon feeds in the presence of steam (“steam cracking”) is a commercially important technology for producing light olefins such as ethylene, propylene, and butadiene. Typical hydrocarbon feeds include, e.g., one or more of ethane and propane, naphtha, heavy gas oils, crude oil, etc. Steam cracking furnaces for carrying out steam cracking generally include a convection section, a radiant section located downstream of the convection section, and a quenching stage located downstream of the radiant section. Typically, at least one burner is included in the steam cracking furnace for providing heat to the convection and radiant sections. The burners are typically located in at least one firebox, the firebox being proximate to the radiant section, with the convection section being located downstream of the radiant section with respect to the flow of heated gases (typically combustion gases) produced by the burner. Tubular coils are utilized for conveying the hydrocarbon feed, steam, and mixtures thereof through the furnace's convection and radiant sections.
At the start of the process, a hydrocarbon feed is introduced into one or more of the convection section's tubular coils (the “convection coils”). The convection coils' external surfaces are exposed to the heated gases conducted away from the burner. Heat is indirectly transferred from the heated gases to the hydrocarbon feed for preheating the hydrocarbon feed. Steam is combined with the pre-heated hydrocarbon feed to produce a hydrocarbon+steam mixture. Additional convection coils are utilized for pre-heating the hydrocarbon+steam mixture, e.g., to a temperature at or just below the temperature at which significant thermal cracking occurs.
The preheated hydrocarbon+steam mixture is conducted via cross-over piping from the convection coils to the radiant coils. The radiant coils are located proximate to the burner, typically within the firebox. The preheated hydrocarbon+steam mixture is indirectly heated in the radiant coils, primarily by the transfer of heat from the burner to the radiant coils' exterior surface, e.g., radiant heat transfer from flames produced in one or more burners located in the firebox, radiant heat transfer from the interior surfaces of the firebox enclosure, convective heat transfer from combustion gases traversing the radiant section, etc.
Heat transferred to the preheated hydrocarbon+steam mixture in the radiant coils results in thermal cracking of at least a portion of the mixture's hydrocarbon to produce a radiant coil effluent comprising light olefin, unreacted steam, and unreacted hydrocarbon feed. Transfer line piping is typically utilized for conveying radiant coil effluent from the radiant section to the quenching stage. When the hydrocarbon feed comprises heavy gas oil, the radiant coil effluent typically has a temperature at the radiant coil outlet (the Coil Outlet Temperature or “COT”) of about 790° C. (1450° F.). For hydrocarbon feeds comprising ethane and/or propane, COT is typically about 900° C. (1650° F.).
Radiant coil effluent is conducted away from the radiant coil outlet for quenching in one or more quenching stages in order to halt the thermal cracking reaction. Quenching is typically carried out in close proximity to the radiant coils to lessen the formation of undesired thermal cracking byproducts. Quenching can be carried out by indirectly transferring heat away from the radiant coil effluent, e.g., using one or more heat exchangers (e.g., quench exchangers). Quench exchangers cool the radiant section against water, and produce quenched radiant coil effluent and high-pressure steam. Quench exchangers are beneficial because the high-pressure steam can be expanded in one or more steam-turbines to produce shaft power. The shaft power can be used for operating compressors, which are typically needed in light olefin separation and recovery stages located downstream of the quenching stage.
When the hydrocarbon feed comprises heavy crude oil and/or heavy gas oil, the radiant coil effluent typically comprises a significant amount of pyrolysis tar, e.g., steam cracker tar (“SCT”). It has been observed that SCT deposits foul internal surfaces of quench exchangers, which lessens the amount of indirect heat transfer from the radiant coil effluent, resulting in less than the desired amount of quenching.
In order to overcome this difficulty, heat is directly transferred from the radiant coil effluent, e.g., by contacting the radiant coil effluent with a hydrocarbon, typically an oil (“quench oil”), having a temperature lower than that of the radiant coil effluent. Quenching can be carried out by directly injecting quench oil into the radiant coil effluent, e.g., by injecting quench oil into a segment of the transfer line piping located in the quenching stage.
Quench oil injection leads to a rapid cooling of the radiant coil effluent, resulting primarily from quench oil vaporization in the quenching stage. A quenched product mixture, comprising radiant coil effluent and vaporized quench oil, is conducted away from the quench stage to one or more separation and recovery stages, e.g., for separating and recovering light olefin from the quenched product mixture. Quench oil can be separated from the quenched product mixture for recycle and re-use in the quenching stage.
Coke is an undesirable byproduct of steam cracking, which forms on internal coil surfaces of the steam cracking furnace, e.g., on the radiant coils' internal surfaces. The presence of coke lessens heat transfer to the preheated hydrocarbon/steam mixture in the radiant coils, which results in less than the desired amount of thermal cracking. The presence of coke can also lead to undesirable changes in radiant coil composition, e.g., as a result of carburization, leading to radiant coil deterioration. Accordingly, it is desirable to remove coke from one or more of the furnace coils during periodic “decoking” modes, during which at least some of the furnace's coils (e.g., all of the furnace's radiant coils) are designated for decoking.
Furnace coil decoking during decoking mode typically includes (i) substituting a flow of air for the flow of hydrocarbon feed to the convection coils, (ii) adjusting the flow of steam to the convection coils and combining the air with the steam to produce a preheated air-steam mixture, (iii) passing the pre-heated air/steam decoking mixture through the cross-over piping from the convection coils to the radiant coils, (iv) substituting a flow of quench water for the flow of quench oil into the quenching stage, and (v) contacting decoking effluent exiting from the radiant coils with the quench water in the quenching stage to quench the decoking effluent. A quenched decoking effluent, comprising decoking effluent and vaporized quench water, is conducted away from the quenching stage, e.g., to one or more decoking separation stages (rather than to the quenched product mixture separation and recovery stages).
Decoking is net exothermic. Additional heat is added to those furnace tubes undergoing decoking. The combination of decoking reaction heat and furnace heat can lead to overheating of furnace components, resulting in damage to the quenching and decoking separation stages during decoking. It is conventional to lessen the effects of overheating during decoking by regulating the amount of quench water injected into the decoking effluent. More particularly, it is desired to regulate temperature in the transfer line piping located downstream of decoking effluent quenching and in piping within the decoking separation stage to a temperature ≦Tmax. Tmax is approximately 840° F. (about 450° C.) when carbon steel piping is utilized in these locations. Quench water flow rate is increased, maintained, or lessened in response to temperature measured at one or more locations in quenching stage conduits and in decoking separation stage conduits, in order to achieve the temperature desired for the transfer line piping located downstream of decoking effluent quenching.
Difficulties have been encountered when regulating the flow of quench water into the quenching stage, resulting in a loss of temperature control and the potential for exceeding Tmax. It is desired to overcome these difficulties.